Diaphragm and centrifugal rotating machine

ABSTRACT

A diaphragm rotatably covers an impeller around an axis. A diaphragm has a suction flow channel configured to supply a gas toward an inlet of an impeller, a diffuser flow channel through which the gas discharged from the impeller outward in a radial direction flows, and a communication section configured to bring the suction flow channel and the diffuser flow channel in constant communication with each other.

TECHNICAL FIELD

The present invention relates to a diaphragm, and a centrifugal rotating machine including the same.

Priority is claimed on Japanese Patent Application No. 2014-020490, filed Feb. 5, 2014, the content of which is incorporated herein by reference.

BACKGROUND ART

For example, a centrifugal compressor is known as a type of centrifugal rotating machine. In the centrifugal compressor, a gas flows in a radial direction of a rotating impeller, and the gas is compressed using a centrifugal force. In this kind of centrifugal compressor, a multi-stage centrifugal compressor in which impellers are provided in multiple stages in an axial direction to compress the gas in stages is known.

Incidentally, in the above-mentioned multi-stage centrifugal compressor, a phenomenon known as surging occurs, in which a gas flows backward from a downstream side toward an upstream side in the impellers, occurs. A method is known in which the occurrence of surging is suppressed by forming a bypass line configured to return some of a main stream from a downstream side to an upstream side of a flow of the main stream, and achieving the enlargement of an operation range, when a flow rate of the entire system is smaller than a flow rate at which such surging occurs, i.e., a surge flow rate.

For example, in Patent Literature 1, an example of such a bypass line is disclosed. The bypass line is formed to recirculate some of the fluid from a discharge side toward a suction side of the impeller of each stage when the impeller of each stage approaches a surging state.

CITATION LIST Patent Literature

[Patent Literature 1]

Japanese Patent No. 2637144

SUMMARY OF INVENTION Technical Problem

However, in the case of the bypass line of Patent Literature 1, only when the bypass line approaches the surging stage, the bypass line is opened to recirculate the fluid.

For this reason, when the system flow rate is slightly larger than the flow rate in which the surging occurs, or the like, recirculation of the fluid is not performed. Accordingly, since the bypass line can reach a state extremely close to the occurrence of the surging without surging occuring, the flow may become instable and cause shaft vibrations. In addition, in the bypass line of Patent Literature 1, since a structure such as a spring or the like configured to open and close the bypass line is installed, a pressure drop when the fluid flows into the bypass line is increased, and compression efficiency is decreased.

In consideration of the above-mentioned circumstances, an object of the present invention is to provide a diaphragm and a centrifugal compressor capable of suppressing the occurrence of surging and enlarging an operation range while maintaining compression efficiency.

Solution to Problem

In order to solve the aforementioned problems, the present invention employs the following means.

That is, a diaphragm of an aspect of the present invention is a diaphragm configured to rotatably cover an impeller about an axis, the diaphragm having: an inlet-side flow channel configured to supply a fluid toward an inlet of the impeller; an outlet-side flow channel through which the fluid discharged from the impeller outward in the radial direction flows; and a communication section configured to bring the inlet-side flow channel and the outlet-side flow channel in constant communication with each other.

According to the above-mentioned diaphragm, as the communication section is formed, the fluid compressed or pumped by the impeller can be constantly recirculated from the outlet-side flow channel to the inlet-side flow channel.

Accordingly, the occurrence of surging can be suppressed by smoothly recirculating the fluid from a downstream side of the impeller to an upstream side of the impeller through the communication section.

In addition, the diaphragm may further include a first vane disposed in the inlet-side flow channel and configured to guide the fluid in a desired direction; and a second vane disposed in the outlet-side flow channel and configured to guide the fluid in a desired direction, wherein the communication section is disposed between a position closer to an upstream side than the first vane and a position closer to a downstream side than the second vane.

As the communication section is disposed at the above-mentioned position, the communication section is disposed at a position spaced apart from the impeller. Accordingly, the fluid recirculated through the communication section is less susceptible to an influence of rotation of the impeller. Accordingly, the fluid can be smoothly recirculated.

Further, a centrifugal rotating machine as another aspect of the present invention includes the above-mentioned diaphragm; and an impeller configured to be supported by the diaphragm to be relatively rotatable around the axis with respect to the diaphragm.

According to the above-mentioned centrifugal rotating machine, as the diaphragm is provided, the fluid can be smoothly circulated from the downstream side of the impeller to the upstream side of the impeller through the communication section for constant communication, and a flow rate of the entire system of the centrifugal rotating machine can be suppressed from approaching a surge flow rate.

In addition, the centrifugal rotating machine may include a plurality of impellers arranged in a direction of the axis and rotated around the axis, wherein the diaphragm supports at least one impeller, among the plurality of impellers, in which a surge flow rate is designed to be mostly increased.

In this way, as the communication section is selectively applied to the impeller in which the surge flow rate is designed to be mostly increased, a stage of the impeller in which recirculation occurs can be limited. Accordingly, since the flow rate of the entire system of the centrifugal rotating machine can be suppressed to reduce power, the occurrence of the surging can be effectively suppressed.

Further, the centrifugal rotating machine may include a plurality of impellers arranged in a direction of the axis and rotated around the axis, wherein a plurality of diaphragms are arranged in the direction of the axis to support the plurality of impellers, respectively, and flow channel areas of the fluid in the communication sections are different in each of the diaphragms.

Since the flow channel areas of the communication sections are different in each of the diaphragms in this way, in combination with the surge flow rates different for each of the impellers of the stages, flow rates of the fluid recirculated from the downstream sides to the upstream sides of the impellers through the communication sections can be adjusted. That is, the flow rate of the recirculated fluid can be adjusted according to specifications of the stages, and the occurrence of the surging can be more effectively suppressed.

Advantageous Effects of Invention

According to the aspect of the present invention, as a communication section in a constant communication state is installed, the occurrence of surging can be suppressed while maintaining compression efficiency and the enlargement of an operation range is possible.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal cross-sectional view including an axis of a multi-stage centrifugal compressor according to an embodiment of the present invention.

FIG. 2 is a graph showing a difference between surge lines of a multi-stage centrifugal compressor according to the embodiment of the present invention and a multi-stage centrifugal compressor when a communication section is not provisionally installed and showing a relation between a flow rate and a compression ratio of a gas required for a system represented by a horizontal axis.

FIG. 3 is a longitudinal cross-sectional view including an axis of a multi-stage centrifugal compressor according to a first variant of the embodiment of the present invention.

FIG. 4 is a longitudinal cross-sectional view including an axis of a multi-stage centrifugal compressor according to a second variant of the embodiment of the present invention.

FIG. 5 is a longitudinal cross-sectional view including an axis of a multi-stage centrifugal compressor according to a third variant of the embodiment of the present invention.

FIG. 6 is a longitudinal cross-sectional view including an axis of a multi-stage centrifugal compressor according to a fourth variant of the embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of a multi-stage centrifugal compressor 1 (a centrifugal rotating machine) according to the present invention will be described with reference to the accompanying drawings.

As shown in FIG. 1, the multi-stage centrifugal compressor 1 includes a rotary shaft 2 that rotates around an axis O, a plurality of impellers 3 attached to the rotary shaft 2, and a casing 4 configured to rotatably support the rotary shaft 2 and having a casing flow channel FC through which a gas G (a fluid) such as air or the like flows.

The rotary shaft 2 has a columnar shape extending along the axis O and formed about the axis O. The rotary shaft 2 is relatively rotated with respect to the casing 4 around the axis O by a power source such as an electric motor or the like (not shown).

The plurality of impellers 3 are disposed at intervals in the axis O direction in which the axis O extends. In the multi-stage centrifugal compressor 1 of the embodiment, five impellers 3 are arranged.

Hereinafter, the impellers 3 are a first stage impeller 3 a, a second stage impeller 3 b, a third stage impeller 3 c, a fourth stage impeller 3 d and a fifth stage impeller 3 e that are disposed from the upstream side to the downstream side through which the gas G flows (from one side to the other side in the axis O direction).

Each of the impellers 3 has a disk-shaped hub 11 having a diameter that is gradually increased toward the downstream side in the axis O direction, a plurality of blades 12 radially attached to the hub 11 and lined up spaced apart from each other in a circumferential direction with respect to the axis O, and a shroud 13 attached to cover the plurality of blades 12 from the upstream side in the axis O direction.

A region surrounded by the blades 12 neighboring in the circumferential direction, the hub 11 and the shroud 13 is an impeller flow channel FC0 through which the gas G flows. An inlet into which a gas is suctioned and an outlet through which the gas is discharged are formed at the impeller flow channel FC0. The inlet is formed at an upstream end in the axis O direction of the impeller flow channel FC0. The outlet is formed at an outer end in the radial direction that is a portion of a downstream side in the axis O direction of the impeller flow channel FC0.

Further, the impeller 3 may be a closed impeller at which the shroud 13 is installed as in the embodiment, or may be an open impeller at which the shroud 13 is not installed unlike the embodiment.

The casing flow channel FC through which the gas G flows is formed in the casing 4. The gas G is compressed by a centrifugal force as the gas G flows from the impeller flow channel FC0 of the first stage impeller 3 a to the impeller flow channel FC0 of the fifth stage impeller 3 e via the casing flow channel FC in stages.

The casing 4 has journal bearings 7 installed at both ends in the axis O direction of the rotary shaft 2, and a thrust bearing 8 installed at an end portion of one side. The casing 4 supports the rotary shaft 2 using the journal bearings 7 and the thrust bearing 8. The rotary shaft 2 is relatively rotatably supported with respect to the casing 4.

The casing flow channel FC of the casing 4 is formed in the casing 4 in an annular shape about the axis O.

The casing flow channel FC has a suction flow channel FC1 (an inlet-side flow channel) configured to bring the inlet of the impeller flow channel FC0 in the first stage impeller 3 a and the outside of the multi-stage centrifugal compressor 1 in communication with each other, a discharge flow channel FC2 (an outlet-side flow channel) configured to bring the outlet of the impeller flow channel FC0 of the fifth stage impeller 3 e and the outside of the multi-stage centrifugal compressor 1 in communication with each other, and intermediate flow channels FC3 respectively formed between the impellers 3 of the stages.

The suction flow channel FC1 is formed in the casing 4 at a position closer to the upstream side in the axis O direction than the first stage impeller 3 a. The suction flow channel FC1 is opened outward in the radial direction at a portion in the circumferential direction of the casing 4 and extends inward in the radial direction, and the gas G is supplied to the inlet of the impeller flow channel FC0 in the first stage impeller 3 a.

An inlet guide vane 21 (a first vane) configured to turn the gas G suctioned from the outside of the multi-stage centrifugal compressor 1 to a desired direction to guide the gas G into the impeller flow channel FC0 is installed in the suction flow channel FC1. The inlet guide vane 21 can adjust an inclination in the circumferential direction with respect to the radial direction using an operation mechanism (not shown). The desired direction is, for example, a direction inclined forward in a rotational direction of the impeller 3 with respect to the radial direction to apply prerotation to the gas G suctioned from the outside.

The discharge flow channel FC2 is formed in the casing 4 to extend from the outlet of the impeller flow channel FC0 in the fifth stage impeller 3 e outward in the radial direction. An outlet configured to discharge the gas G is formed at the discharge flow channel FC2. An outlet of the discharge flow channel FC2 is opened at a portion in the circumferential direction of the casing 4 outward in the radial direction. The discharge flow channel FC2 allows the gas discharged from the outlet of the impeller flow channel FC0 in the fifth stage impeller 3 e to flow therethrough to be discharged to the outside.

A discharge scroll S serving as a space extending in an annular shape in the circumferential direction is formed at the discharge flow channel FC2 at a position in front of the outlet of the discharge flow channel FC2. The discharge scroll S increases a pressure of the gas G discharged from the outlet of the impeller flow channel FC0 of the fifth stage impeller 3 e.

Here, in the embodiment, as shown by a broken line of FIG. 1, a diffuser vane 22 (a second vane) may be formed between the discharge scroll S and the impeller 3 in the discharge flow channel FC2. The diffuser vane 22 turns the gas G discharged from the impeller flow channel FC0 to a desired direction to guide the gas G to the discharge scroll S and converts a dynamic pressure of the flowing gas G into a static pressure. The desired direction is a direction in which conversion to the static pressure is performed, i.e., a direction inclined in the circumferential direction with respect to the radial direction.

The intermediate flow channels FC3 are formed in the casing 4 at a position between the first stage impeller 3 a and the second stage impeller 3 b, a position between the second stage impeller 3 b and the third stage impeller 3 c, a position between the third stage impeller 3 c and the fourth stage impeller 3 d, and a position between the fourth stage impeller 3 d and the fifth stage impeller 3 e.

Since all of the intermediate flow channels FC3 between the stages have substantially the same configuration, the intermediate flow channel FC3 between the first stage impeller 3 a and the second stage impeller 3 b will be representatively described.

The intermediate flow channel FC3 is formed in the casing 4 without communication with the outside of the casing 4. The intermediate flow channel FC3 has a diffuser flow channel FC4 (an outlet-side flow channel) extending outward in the radial direction from the outlet of the impeller flow channel FC0 in the first stage impeller 3 a, and a return flow channel FC5 connected to the diffuser flow channel FC4 and extending toward the inlet of the impeller flow channel FC0 of the second stage impeller 3 b.

The gas G discharged from the impeller flow channel FC0 of the first stage impeller 3 a outward in the radial direction flows through the diffuser flow channel FC4. The above-mentioned diffuser vane 22 (the second vane) may be installed in the diffuser flow channel FC4.

The return flow channel FC5 is constituted by a first bending flow channel section FC6 connected to an end portion outside in the radial direction of the diffuser flow channel FC4, a linear flow channel section FC7 connected to an end portion of the first bending flow channel section FC6, and a second bending flow channel section FC8 connected to an end portion of the linear flow channel section FC7.

The first bending flow channel section FC6 extends from the diffuser flow channel FC4 outward in the radial direction and then is curved inward in the radial direction. The first bending flow channel section FC6 turns a flow of the gas G from the impeller flow channel FC0 of the first stage impeller 3 a outward in the radial direction into a flow inward in the radial direction.

The linear flow channel section FC7 is connected to a radially inner end of the first bending flow channel section FC6, which is an end opposite to a connecting portion between the diffuser flow channel FC4 and the first bending flow channel section FC6. The linear flow channel section FC7 extends from the first bending flow channel section FC6 inward in the radial direction.

A return vane 23 (a first vane) configured to turn the gas G from the impeller flow channel FC0 of the first stage impeller 3 a into a desired direction and guide the gas G to the impeller flow channel FC0 of the second stage impeller 3 b is formed in the linear flow channel section FC7. The desired direction is, for example, a direction in which a swirling component of the gas G from the impeller flow channel FC0 of the first stage impeller 3 a is removed, i.e., a direction inclined rearward in the rotational direction of the impeller 3 with respect to the radial direction.

The second bending flow channel section FC8 is connected to a radially inner end portion of the linear flow channel section FC7 and is curved from the end portion along the other side (a downstream side) of the axis O. The second bending flow channel section FC8 turns a flow of the gas G from the linear flow channel section FC7 into a flow toward the impeller flow channel FC0 of the second stage impeller 3 b.

Here, in the embodiment, the casing flow channel FC further includes a communication section 24 configured to bring the diffuser flow channel FC4 extending outward in the radial direction from the impeller flow channel FC0 of the first stage impeller 3 a and the suction flow channel FC1 in constant communication with each other.

In the embodiment, the communication section 24 is constituted by a plurality of communication holes formed at intervals in the circumferential direction.

Further, a shape of the communication hole is not particularly limited but may be a circular cross-sectional shape or a polygonal cross-sectional shape. In addition, the communication hole may have a slit shape. That is, the communication hole may have any shape as long as the communication hole brings the diffuser flow channel FC4 and the suction flow channel FC1 in constant communication with each other, or may be formed at only one place in the circumferential direction.

Further, when the diffuser vane 22 is formed in the diffuser flow channel FC4, the communication section 24 may come in communication with an outer side further in the radial direction than the diffuser vane 22, i.e., a downstream side of a flow of the gas G, and an outer side further in the radial direction than the inlet guide vane 21, i.e., an upstream side of the flow of the gas G.

Here, the suction flow channel FC1 and a portion of the casing 4 in which the diffuser flow channel FC4 formed at the outlet side of the impeller flow channel FC0 of the first stage impeller 3 a is formed constitute a first stage diaphragm 4 a.

In addition, the return flow channel FC5 between the first stage impeller 3 a and the second stage impeller 3 b, and a portion of the casing 4 in which the diffuser flow channel FC4 formed at the outlet side of the impeller flow channel FC0 of the second stage impeller 3 b is formed constitute a second stage diaphragm 4 b.

As with the second stage diaphragm 4 b, a third stage diaphragm 4 c and a fourth stage diaphragm 4 d are similarly defined.

Further, the return flow channel FC5 between the fourth stage impeller 3 d and the fifth stage impeller 3 e, and a portion of the casing 4 in which the discharge flow channel FC2 is formed constitute a fifth stage diaphragm 4 e.

In the embodiment, the communication section 24 is formed at the first stage diaphragm 4 a, and in the embodiment, a surge flow rate when the surging occurs is designed to have a maximum value at the first stage impeller 3 a.

According to the above-mentioned multi-stage centrifugal compressor 1, the communication section 24 is formed at the first stage diaphragm 4 a. For this reason, some of the gas G compressed by the first stage impeller 3 a can be constantly recirculated from the diffuser flow channel FC4 to the suction flow channel FC1, i.e., the gas G can be constantly recirculated from a downstream side of the first stage impeller 3 a at a high pressure to an upstream side at a low pressure by differential pressure.

Here, provided that a flow rate of the gas G flowing through the impellers 3 of the stages is 100 when some of the gas G is not recirculated without forming the communication section 24, a total flow rate of the gas G flowing through the impellers 3 of the stages is 500.

In addition, when some of the gas G is not circulated without forming the communication section 24, provided that a surge line L0 is assumed to be as shown by a broken line of FIG. 2, an operating point A is disposed closer to a small flow rate side than the surge line.

In such a circumstance, as the surging occurs when an operation of the multi-stage centrifugal compressor 1 is performed at the operating point A, a stable operation cannot be performed.

Meanwhile, when the communication section 24 is formed as in the embodiment and a flow rate of the gas G recirculated through the communication section 24 is 10, the surge line L0 appears to be shifted 10% to the small flow rate side, and becomes a surge line L (a solid line of FIG. 2). Accordingly, as shown in FIG. 2, when a flow rate of the operating point A is larger than that of the surge line L, a stable operation becomes possible.

Further, when a flow rate of 10 of the gas G for a recirculation amount is added, since a total flow rate of the gas G flowing through the impellers 3 of the stages is 500, power is increased by the amount of the flow rate of 10 of the gas G in comparison with the case in which the recirculation is not performed. However, in the system of provisionally recirculating the gas G through all of the stages of the system from the downstream side of the fifth stage impeller 3 e to the upstream side of the first stage impeller 3 a, a flow rate of the gas G flowing through the impellers 3 of the stages is 110. Accordingly, power for allowing an amount of a total flow rate of 550 of the gas G to flow through the impellers 3 of the stages is required.

Accordingly, as in the embodiment, as the communication section 24 is selectively applied to the impeller 3 designed to have a largest surge flow rate and some of the gas G is circulated through the downstream side and the upstream side of the impeller 3 of the stage in which the surge flow rate is largest upon design, since the flow rate of the gas G of the entire system of the multi-stage centrifugal compressor 1 can be suppressed to reduce the power, occurrence of the surging can be effectively suppressed.

In addition, in the case in which the diffuser vane 22 is provisionally installed, when the communication section 24 brings an outer side further in the radial direction than the diffuser vane 22 in communication with an outer side further in the radial direction than the inlet guide vane 21, the communication section 24 is disposed at a position spaced apart farther from the first stage impeller 3 a. Accordingly, the gas G recirculated through the communication section 24 is less susceptible to an influence of rotation of the first stage impeller 3 a. Accordingly, the gas G can be smoothly recirculated.

In addition, when the diffuser vane 22 is provisionally installed in this way, after the gas G passing through the first stage impeller 3 a is restored to the static pressure by the diffuser vane 22, some of the gas G can flow into the communication section 24. Accordingly, the gas G can easily enter the communication section 24, and the fluid can be stably and smoothly circulated to the upstream side of the impeller 3 through the communication section 24.

Further, since the communication section 24 is a simple communication hole or slit, there is no member to block the flow of the gas G in the communication section 24, a pressure drop upon recirculation can be suppressed to a low level, and the gas G can be smoothly recirculated.

According to the multi-stage centrifugal compressor 1 of the embodiment, as the communication section 24 for constant communication is provided, the occurrence of surging can be suppressed while suppressing an increase in power of the entire system of the multi-stage centrifugal compressor 1 to a minimum level.

Further, the communication section 24 of the embodiment may not bring an outer side further in the radial direction than the diffuser vane 22 in communication with an outer side further in the radial direction than the inlet guide vane 21.

That is, the communication section 24 may be formed to bring at least the downstream side and the upstream side of the first stage impeller 3 a in communication with each other.

Here, when the surge flow rate is provisionally designed to be maximally increased at the second stage impeller 3 b, as shown in FIG. 3, the communication section 24 is formed at the second stage diaphragm 4 b and may bring the downstream side and the upstream side of the second stage impeller 3 b in communication with each other. Even in this case, the occurrence of surging can be suppressed while suppressing an increase in power of the entire system of the multi-stage centrifugal compressor 1 to a minimum level.

Similarly, when the surge flow rate is provisionally designed to be maximally increased at the third stage impeller 3 c, the communication section 24 may be formed at the third stage diaphragm 4 c.

In addition, when the surge flow rate is designed to be maximally increased at the fourth stage impeller 3 d, the communication section 24 may be formed at the fourth stage diaphragm 4 d.

Further, when the surge flow rate is provisionally designed to be maximally increased at the fifth stage impeller 3 e, the communication section 24 may be configured as shown in FIG. 4. Specifically, the communication section 24 may be formed to bring an outer side further in the radial direction than the diffuser vane 22 of the outlet side of the flow rate of the impeller 3 of the fifth stage impeller 3 e and an outer side further in the radial direction than the return vane 23 of the inlet side of the flow rate of the impeller 3 of the fifth stage impeller 3 e in communication with each other. Then, as shown in FIG. 4, the communication section 24 may come in communication with the discharge scroll S.

In addition, when the surge flow rate is provisionally designed to be maximally increased at the fourth stage impeller 3 d and the fifth stage impeller 3 e, as shown in FIG. 5, the communication section 24 may be configured as shown in FIG. 5. Specifically, the communication section 24 is formed to bring an outer side further in the radial direction than the diffuser vane 22 of the outlet side of the flow rate of the impeller 3 of the fifth stage impeller 3 e and an outer side further in the radial direction than the return vane 23 of the inlet side of the flow rate of the impeller 3 of the fourth stage impeller 3 d in communication with each other. That is, in this case, it is also considered that the fourth stage diaphragm 4 d and the fifth stage diaphragm 4 e constitute a diaphragm of the first stage.

Further, FIG. 6 shows another variant of the embodiment. In the variant, communication sections 24 are formed at the diaphragms 4 a to 4 e of all of the stages. That is, the communication section 24 is formed to bring the downstream sides and upstream sides of the impellers 3 of all of the stages in communication with each other.

In addition, the communication sections 24 have different flow channel areas, through which the gas G flows, according to the diaphragms 4 a to 4 e of the stages. For example, while hole diameters are different from each other according to the diaphragms 4 a to 4 e when the communication sections 24 are the communication holes, the number of communication holes can be different.

According to the above-mentioned multi-stage centrifugal compressor 1, as the flow channel areas of the communication sections 24 are different in each of the diaphragms 4 a to 4 e, in combination with the surge flow rates that are different according to the impellers 3 of the stages, a flow rate of the gas G recirculated from the downstream sides to the upstream sides of the impellers 3 through the communication sections 24 can be adjusted.

That is, the flow rate of the gas G recirculated according to specifications of the stages can be adjusted, and it is possible to approach a state in which surge occurring at all of the stages is eliminated. Accordingly, the occurrence of the surging can be more effectively suppressed.

Hereinabove, while the embodiments of the present invention have been described in detail, some design modifications may be made without departing from the technical spirit of the present invention.

For example, in the above-mentioned embodiment, while the multi-stage centrifugal compressor 1 has been described as an example of the centrifugal rotating machine, the diaphragm of the above-mentioned embodiment may be applied to another centrifugal rotating machine such as a multi-stage centrifugal pump or the like configured to pump a liquid instead of the gas G.

INDUSTRIAL APPLICABILITY

According to the aspects of the present invention, occurrence of surging can be suppressed while maintaining compression efficiency.

REFERENCE SIGNS LIST

1 Multi-stage centrifugal compressor (rotating machine)

2 Rotary shaft

3 Impeller

G Gas (fluid)

3 a First stage impeller

3 b Second stage impeller

3 c Third stage impeller

3 d Fourth stage impeller

3 e Fifth stage impeller

4 Casing

4 a First stage diaphragm

4 b Second stage diaphragm

4 c Third stage diaphragm

4 d Fourth stage diaphragm

4 e Fifth stage diaphragm

7 Journal bearing

8 Thrust bearing

11 Hub

12 Blade

13 Shroud

21 Inlet guide vane (first vane)

22 Diffuser vane

23 Return vane (first vane)

24 Communication section

FC Casing flow channel

FC0 Impeller flow channel

FC1 Suction flow channel (inlet-side flow channel)

FC2 Discharge flow channel (outlet-side flow channel)

FC3 Intermediate flow channel

FC4 Diffuser flow channel (outlet-side flow channel)

FC5 Return flow channel (inlet-side flow channel)

FC6 First bending flow channel section

FC7 Linear flow channel section

FC8 Second bending flow channel section

S Discharge scroll

O Axis 

1. (canceled)
 2. A diaphragm configured to rotatably cover an impeller about an axis, the diaphragm having: an inlet-side flow channel configured to supply a fluid toward an inlet of the impeller; an outlet-side flow channel through which the fluid discharged from the impeller outward in the radial direction flows; a communication section configured to bring the inlet-side flow channel and the outlet-side flow channel in constant communication with each other; a first vane disposed in the inlet-side flow channel and configured to guide the fluid in a desired direction; and a second vane disposed in the outlet-side flow channel and configured to guide the fluid in a desired direction, wherein the communication section is disposed between a position closer to an upstream side than the first vane and a position closer to a downstream side than the second vane.
 3. A centrifugal rotating machine comprising: the diaphragm according to claim 2; and an impeller configured to be supported by the diaphragm to be relatively rotatable around the axis with respect to the diaphragm.
 4. The centrifugal rotating machine according to claim 3, comprising a plurality of impellers arranged in a direction of the axis and rotated around the axis, wherein the diaphragm supports at least one impeller, among the plurality of impellers, in which a surge flow rate is designed to be mostly increased.
 5. The centrifugal rotating machine according to claim 3, comprising a plurality of impellers arranged in a direction of the axis and rotated around the axis, wherein a plurality of diaphragms are arranged in the direction of the axis to support the plurality of impellers, respectively, and flow channel areas of the fluid in the communication sections are different in each of the diaphragms. 